Sphingosine 1-phosphate (SIP) is a lysophospholipid mediator that evokes a variety of cellular responses by stimulation of five members of the endothelial cell differentiation gene (EDG) receptor family. The EDG receptors are G-protein coupled receptors (GPCRs) and on stimulation propagate second messenger signals via activation of heterotrimeric G-protein alpha (Gα) subunits and beta-gamma (Gβγ) dimers.
Sphingosine 1-phosphate (S1P) evokes many responses from cells and tissues. Prominent among these are resistance to apoptosis, changes in cell morphology, cell migration, cell division, angiogenesis and modulation of the immune system via alterations of lymphocyte trafficking. Therefore, S1P receptors are targets for therapy of, for example, neoplastic diseases, autoimmune disorders and rejection of tissue allografts. Sphingosine-1-phosphate signals cells in part via a set of G protein-coupled receptors named S1P1, S1P2, S1P3, S1P4, and S1P5. These receptors share 50-55% identical amino acids and cluster with three other receptors (LPA1, LPA2, and LPA3) for the structurally related lysophosphatidic acid (LPA).
A conformational shift is induced in the G-Protein Coupled Receptor (GPCR) when the ligand binds to that receptor, causing GDP to be replaced by GTP on the α-subunit of the associated G-proteins and subsequent release of the G-proteins into the cytoplasm. The α-subunit then dissociates from the βγ-subunit and each subunit can then associate with effector proteins, which activate second messengers leading to a cellular response. Eventually the GTP on the G-proteins is hydrolyzed to GDP and the subunits of the G-proteins re-associate with each other and then with the receptor. Amplification plays a major role in the general GPCR pathway. The binding of one ligand to one receptor leads to the activation of many G-proteins, each capable of associating with many effector proteins leading to an amplified cellular response.
S1P receptors make good drug targets because individual receptors are both tissue and response specific. Tissue specificity of the S1P receptors is desirable because development of an agonist or antagonist selective for one receptor localizes the cellular response to tissues containing that receptor, limiting unwanted side effects. Response specificity of the S1P receptors is also of importance because it allows for the development of agonists or antagonists that initiate or suppress certain cellular responses without affecting other responses. For example, the response specificity of the S1P receptors could allow for an S1P mimetic that initiates platelet aggregation without affecting cell morphology.
Sphingosine-1-phosphate is formed as a metabolite of sphingosine in its reaction with sphingosine kinase and is stored in abundance in the aggregates of platelets where high levels of sphingosine kinase exist and S1P lyase is lacking. S1P is released during platelet aggregation, accumulates in serum, and is also found in malignant ascites. Reversible biodegradation of S1P is believed to proceed via hydrolysis by ectophosphatases such as the S1P phosphohydrolase, S1P is degraded irreversibly by S1P lyase.
The physiologic implications of stimulating individual S1P receptors are largely unknown due in part to a lack of receptor type selective ligands. Isolation and characterization of S1P analogs that have potent agonist or antagonist activity for S1P receptors has been limited due to the complication of synthesis derived from the lack of solubility of S1P analogs.
Currently, there is a need for novel, potent, and selective agents which are agonists of the S1P receptor. There is also a need for pharmacological tools for the further study of the physiological processes associated with agonism of the S1P receptors.